The invention relates generally to the field of marine electromagnetic surveying of subsurface rock formations. More specifically, the invention relates to electrical configurations for electromagnetic sensor cables used in such surveying.
Marine electromagnetic geophysical surveying is used to infer spatial distribution of electrical conductivity of rock formations below the bottom of a body of water such as a lake or ocean. The spatial distribution of conductivity is used to assist determining presence of hydrocarbon bearing rock formations in the subsurface. One type of such surveying generally includes inducing a time varying electromagnetic (“EM”) field in the subsurface formations and measuring one or more parameters related to a response of the subsurface rock formations to the induced EM field.
Devices for inducing such EM fields are generally referred to as “sources” or “transmitters” and include, among other devices, spaced apart electrodes disposed along or at the end of a cable. The cable may be towed by a vessel in the body of water. Time varying electric current is imparted across the electrodes, generally from a power source located on the vessel, to induce a time varying EM field in the water and subsequently in the subsurface formations. The electrodes may be suspended at a selected depth in the water by the use of floatation devices such as buoys, or the cable itself may be neutrally or otherwise buoyant.
The response of the subsurface formations below the water bottom may be detected by various sensors on long cables or “streamers” towed in the water behind the survey vessel or a different vessel. In some examples, the streamer includes pairs of spaced apart electrodes to detect an electric field component of the EM field response.
The direct EM field strength decreases rapidly with respect to distance from the EM field source in an EM measurement system. The corresponding EM field modulated by the subsurface rock formations decreases even more rapidly with respect to distance from the transmitter or field source. When electrode pairs are used to detect the electric field component of the EM field, such pairs need to have a short separation between the elements of the pair when the pair is disposed close to the source in order to not saturate an input amplifier typically associated with the electrode pair. At long distances (“offsets”) from the source the electrodes in respective pairs need to be separated by a greater distance in order to be able to measure the weaker electric field component.
Marine streamers are typically assembled from segments each about 75 meters length, and may include a number of such interconnected segments such that the total streamer length may be several kilometers. “Short” electrode pairs can typically be fitted within a standard marine geophysical streamer segment length of 75 in, while longer electrode pair lengths can be many times longer than the length of a typical streamer segment. The spacing requirements for near offset and long offset electrode pairs are thus contradictory with respect to designing a single streamer wiring configuration.
What is needed is a wiring configuration that can be used in a marine sensor streamer having selectable spacing between respective pairs of electrodes.